The proposed work will include neuropsychological examination of new patients and systematic follow-up studies of 1) previously tested patients with hemispherectomy for intractable seizures (in infantile hemiplegics) and for brain tumor; 2) commissurotomy; and 3) subtotal agenesis of either hemisphere. Comparisons of preoperative and/or initial and later postoperative studies of these patients provide data indicating principles underlying the organization, disorganization and reorganization of cerebral functions following pre-, and post-natal insults at different stages in the development of the brain including: 1) the nature, rate and extent of "functional plasticity" of the brain with advancing age, 2) the extent to which each hemisphere participates and specializes in certain language and nonlanguage functions as the brain matures, 3) Limitations and principles underlying the reorganization of cerebral functions following hemispherectomy, commissurotomy or subtotal agenesis of either hemisphere.